Quartz crystal microbalances used to sense small mass changes are well known in the prior art. Typical microbalances based on quartz technology are described in a treatise by C. Lu and A. W. Czanderna, Applications of Piezoelectric Ouartz Microbalances, Elsevier, (1984). Microbalances are usually used to sense changes in mass during thin film and vapor deposition fabrication of solid-state electronic devices to ensure that such devices are fabricated according to specified tolerances. Microbalances are also used to sense absorbates and warn of chemical contamination.
A microbalance typically consists of an AT-cut or BT-cut crystal resonator. Mass added to or removed from the resonator results in a frequency change. The change in mass can be calculated from this frequency change. For small changes in mass, the frequency change is linearly proportional to the change in mass, provided that the temperature remains constant during deposition.
Although the frequency of a quartz crystal resonator is highly sensitive to changes in mass, it is also sensitive to changes in temperature. Thus, the frequency change measured by a microbalance is effected by both changes in mass and temperature. Resonators used in conventional microbalances, such as those employing an AT-cut, have a zero temperature coefficient at only two temperatures called the "turnover temperatures." Consequently, the further away the operating temperature of a microbalance is from the nearest turnover temperature, the more sensitive is the microbalance to temperature changes, i.e., the larger is the uncertainty in the mass change indicated by the microbalance.
Conventional methods used to control the effects of temperature on a microbalance include: 1) controlling the temperature of the crystal by cooling it; 2) attaching a thermocouple to the crystal to measure the frequency vs. temperature characteristics and then compensating for the temperature effects; 3) using two identical crystals, one of which is exposed to mass changes, the other of which is not; 4) forming two resonators on one crystal plate and exposing only one of them to the mass change; 5) using multiple resonators to allow compensation for different quantities, such as mass, temperature, stress, etc.; and 6) using an "electrode-tab" resonator, on which at least one additional single electrode, the "tab", is deposited with the additional mass only being deposited on the tab. All of these methods suffer from the drawbacks of being cumbersome and inaccurate. For example, the sixth method listed has the disadvantage of a mass range limitation. Moreover, the electrode-tab microbalance is not easily reproduced because the slight differences in the electrode-gap-tab geometries may particularly effect the mass dependence of the resonant frequencies. Therefore, the mass coefficients obtained from earlier calibrations may differ from later ones. Further, with all these methods it is very difficult to determine the temperature of the crystal itself because all these methods require a separate determination of the temperature of the crystal which requires measuring the temperature where the temperature sensor is mounted at some distance from the crystal. Since it is nearly impossible to avoid spatial temperature variations, especially at higher temperatures, the observed temperature generally differs from the temperature of the crystal. Chapter 5 of the treatise by C. Lu and A. W. Czanderna, Applications of Piezoelectric Quartz Micro-balances, Elsevier, (1984), cited above, discusses various ways prior art devices have attempted to solve the temperature dependency of microbalances based on quartz crystal technology.
Accordingly, in the microbalance art, there exists a need to eliminate this temperature dependency problem. This invention addresses such a need.